The battery grip gives you the option of :
- extended battery life with additional battery
- better grip in a way especially with portrait orientation shooting
- possibly giving a better balance especially on longer heavier lenses
- gives u option of using 6 AA batteries if somehow ur original Nikon batts ran out of juice..
* on certain models, they can give u faster fps
You have to see if u really need the additional battery grip that will bulk up things, else really can do without it with ur existing setup. A needless battery grip kinda of defeats the purpose of having a smaller and lighter camera
It's up to your way of shooting. If you shoot portrait mode alot then a battery grip can help you to hold your camera properly without doing chicken wing. Help you to have better control/balance in holding your camera especially if you hand held long telephoto lenses. Plus point you get extra battery life.
For TS case, I don't think you need the battery grip since both lenses you own are quite light weighed. Unless you shoot alot of events, then having a battery grip is preferred.
I'll leave home my battery grip when I know I going shoot landscape or anything that I can take my own sweet time to shoot.